In recent years it has become desirable to reduce emissions from internal combustion engines. Numerous environmental regulations have been enacted mandating that engine manufacturers take steps necessary to reduce environmentally harmful emissions. One common method to reduce engine emissions is to employ external exhaust gas recirculation (EGR) wherein a portion of the exhaust gases produced by the engine are reintroduced into the intake of the engine for further combustion. In order to drive external EGR to flow it is necessary to create a higher pressure in the exhaust manifold than in the intake manifold. This is not a natural trend for internal combustion engines and particularly turbocharged diesel engines wherein the intake manifold pressure is frequently higher than the exhaust pressure. In order to create a higher pressure in the exhaust manifold compared to the intake manifold, a condition known as negative delta P, it is typically necessary to provide additional hardware to the basic engine. The objective of this hardware is to create a restriction someplace in the exhaust system so as to significantly raise the pressure in the exhaust manifold. One currently favored approach for accomplishing this is to employ a variable geometry turbocharger. These devices can be closed down to effectively restrict the flow of gases from the engine thereby inducing a buildup of pressure in the exhaust manifold. The variable geometry turbocharger is able to accomplish this task, albeit at a significantly added product cost and with high concern for reliability problems.
Another problem in creating the needed negative delta P is that the engine naturally develops a high level of positive delta P at full load and lower engine speed (peak torque). Accordingly, the amount of restriction required in the exhaust system at peak torque must be very high to induce negative delta P. If a fixed restriction device is employed for the peak torque condition the back pressure created at full load rated speed will be excessively high and will significantly degrade the fuel economy of the engine at full load rated speed.
Accordingly, there is a clear need in the art for an exhaust restriction device which is capable of creating the needed backpressure at peak torque, but not excessively penalizing fuel economy due to excessive back pressure at the rated speed full load condition.